Flexible armoured pipe and use of same

ABSTRACT

An armoured flexible pipe consists of an inner liner ( 3 ), which surrounds a carcass ( 1, 2 ). On the other side of the inner liner ( 3 ) there are two armour layers, where the innermost ( 5, 6 ) is a pressure armour layer, while the outermost ( 7, 8 ) is a tensile armour layer. An outer sheath ( 9 ) is finally applied around the armour layers. With the object of ensuring a low transport of heat through the walls of the pipe, one or more thermally-insulating layers ( 10 ) are extruded on the outside of the inner liner, where, depending on the use of the pipe, the layers can consist, for example, of a polymer or polymeric mixture, polyolefin, such as a polypropylene, or a polyketon. The flexible pipe according to the invention is especially applicable for use in the extraction, transport or refining of mineral oil or related fluids.

The invention concerns an armoured flexible pipe comprising an innerliner which surrounds a carcass, and where on the outer side of theinner liner there is a pressure armour layer and a tensile armour layerwhich are shielded from the surroundings by an outer sheath, and furthercomprising a thermally-insulating layer on the outside of the innerliner.

The invention also concerns a use of the pipe.

Pipes of the above-mentioned type normally comprise an inner liner,which forms a barrier against the outflow of the fluid, which-isconveyed through the pipe. The inner liner is wound with one or morearmour layers which are not chemically bound to the inner liner butwhich can move in relation thereto, which ensures the flexibility of thepipe during laying out and operation.

Around the armour layers an outer sheath is provided with the object offorming a barrier against the ingress of fluids from the pipesurroundings to the armour layers. In order to prevent the collapse ofthe inner liner, this is often provided on the inner side with aflexible, wound pipe, a so-called carcass.

The above-mentioned type of flexible pipes is used, among other things,for the transport of fluids and gases in different depths of water. Theyare used especially in situations where very high or varying waterpressure exists along the longitudinal axis of the pipe. As examples canbe mentioned riser pipes which extend from the seabed up to aninstallation on or near the surface of the sea. Pipes of this type arealso used between installations, which are located at great depths onthe seabed, or between installations near the surface of the sea.

The armour layers, which are used as pressure armour, are most oftenconstructed in such a way that they comprise different metallicprofiles. When wound with a large angle in relation to the longitudinalaxis of the pipe, these profiles will be able to absorb radial forcesresulting from outer or inner pressure on the pipe. Among other thingsthe profiles thus prevent the pipe from collapsing or exploding as aresult of pressure, and are thus called pressure-resistant profiles.

Conversely, profiles, more specifically tensile armour layers, which arewound with a small angle in relation to the longitudinal axis of thepipe, will not be able to absorb radial forces to any significantdegree, but on the other hand are able to absorb forces exerted alongthe longitudinal axis of the pipe. In the following, this type ofprofiles is referred to as tension-resistant profiles.

Together, the tension-resistant profiles and the pressure-resistantprofiles form the armour for the pipe. In this armouring layer a freevolume of such configuration also exists that this can be ventilated,whereby a destructive build-up of pressure as a result of diffusion doesnot arise.

A problem in connection with the use of pipes of the type describedabove is that the transport of heat through the walls of the pipe can bequite considerable. With certain uses this is critical, since this typeof pipe is often used to transport fluids, which are desired to be heldat a temperature, which deviates from that of the surroundings. As anexample of such a use can be mentioned that of transporting crude oilbetween two installations. If the temperature of the crude oil fallsbelow a certain critical limit, mineral wax and solid hybrids can beformed in the pipe, which results in stoppages in the pipe.

In order to hold the transport of heat through the walls of the pipe atan acceptable level, it is known to wind one or more layers of bandsmade of a so-called syntactic foam on the outside of the pipe'stension-resistant armour, but on the inside of the outer sheath. Thisfoam contains a great amount of hollow glass balls, which have verygreat resistance against crushing, and a polymeric matrix material.Syntactic foam possesses a low heat conductivity coefficient, wherebythe use of this material reduces the transport of heat through the wallsof the pipe to an acceptable level.

However, the use of syntactic foam involves a number of limitations, themost important of which is that the mechanical strength of the foamoften becomes that factor which limits the areas of application of thepipe. The syntactic foam thus possesses very great resistance againsthydrostatic crushing, but only limited resistance against deformationand damage by local mechanical influences. A second problem connectedwith the use of syntactic foam is that the long-term characteristics ofthis material can be problematic to predict.

U.S. Pat. No. 5,934,335 discloses an armoured flexible pipe having aninner liner which surrounds a carcass. The thermally insulating layeraccording to this publication is not intended to protect againstdestructive build-up of gases and condensation in the area between theinner liner and the thermally insulating layer.

It is therefore the object of the present invention to provide a newmethod whereby a sufficiently low transport of heat through the walls ofthe pipe is ensured, and further to ensure that a destructive build-upof gases and condensation does not occur in the area between the innerliner and the extruded layer.

The object of the invention is achieved in that the thermally insulatinglayer is permeable to fluids.

In one disclosed embodiment the thermal insulation consists of one, twoor more layers. A first inner layer can be selected on the basis of thedesire concerning low permeability to the fluid which is transported inthe pipe, while a second inner layer can be selected on the basis of thedesire to ensure a sufficient thermal insulation between the fluid whichis transported through the pipe and the pipe's armour layer.

Preferably, the extruded layer is configured with an adequatepermeability, e.g. by providing the thermal insulation with a number ofsmall holes as disclosed herein.

Further expedient embodiments of the invention are disclosed herein.

As mentioned, the invention also concerns a use of the pipe. This use ofthe flexible pipe comprises use for the extraction, transport, orrefining of mineral oil or related fluids, or for the transport of coldfluids such as, e.g., liquid ammonia.

The invention will now be described in more detail with reference to thedrawing which comprises only one figure, and which shows theconstruction of an armoured flexible pipe according to the inventionwith its various layers.

The figure shows a pipe, which comprises an inner liner 3, whichsurrounds a carcass 1 consisting of a helically wound metal strip 2,which forms an inner pipe. During manufacture, the metal strip 2 isformed with flaps, which engage with each other so that they hereby lockthe individual windings of the metal strip 2 together in such a mannerthat the carcass 1 can be bent in its longitudinal direction.

Since the inner carcass 1 in itself is not impermeable, the purpose ofthe surrounding inner liner 3 is to prevent fluids from flowing to orfrom the inside of the pipe. Since the inner liner is selected on thebasis of the desire concerning low diffusion of gases from the inside ofthe pipe to the armour layer, it is most often not possible at the sametime to produce this from a material which possesses a sufficiently highthermal resistance to ensure the function of the pipe.

Therefore, around the inner liner 3 one or more further layers 10 (thefigure shows one layer) are applied by extrusion. The function of thisor these layers is exclusively to ensure a sufficient thermal insulationbetween the fluid transported inside the pipe and the surrounding armourlayer. There is thus great freedom in the selection of the material ormaterials for these layers, in that regard to permeability to fluids isnot important.

On the outside of the thermally-insulating layer(s) 10, one or morelayers of profiles 5,6 are wound in a helical manner, said profilesforming windings with a large angle in relation to the longitudinaldirection of the pipe. As a consequence of the large angle, the profilesare primarily able to absorb radial forces, which arise as a result ofinner or outer pressure. The inner pressure arises during operation ofthe pipe. The outer pressure arises partly as a result of thehydrostatic pressure of the surroundings, and partly as a result ofmechanical influences during the laying of the pipe. The windings thusform a pressure armour which prevents the inner liner 3 and thethermally-insulating layers from exploding as a result of high pressureon the inside of the pipe, or from collapsing as a result of highpressure on the outside of the pipe.

From FIG. 1 it will also be seen that a tensile armour consisting of oneor several helically wound layers 7,8 is provided on the outside of thepressure armour.

Between the pressure armour and the tensile armour an intermediatesheath (not shown in the figure) can be provided, the purpose of whichis to prevent fluids from migrating between the tensile armour and thepressure armour. These layers are finally surrounded by an outer sheath9.

In addition to its thermal insulation capabilities, the thermallyinsulating layer according to the invention must fulfil a number ofother requirements, which are explained below.

In the design and manufacture of the layer, this must be designed insuch a way that the destructive build-up of fluids in the interfacelayer between the layer and the liner is prevented, in that a suitableand effective transport path from the interface layer to the armourlayer is ensured. There are several methods by which this can beensured, among which can be mentioned:

-   -   by providing the extruded layer with defects which can ensure a        relatively unhindered transport between the interface and the        armour layer,    -   by selecting a thermally-insulating layer with suitably low        barrier characteristics against the gases which may diffuse        through the inner liner,    -   by ensuring a transport path between the interface and the        armour layer at the terminating end of the pipe.

Combinations of the above-mentioned methods could also be used.

Besides, as a consequence of the inner pressure in the pipe, the innerliner and the thermally insulating layer will be able to be pressed outin the armour layer. If the thermally insulating layer is pressed outthrough the armouring strips, this will reduce the flexibility of thepipe to a considerable degree. Therefore, it is important that thethermally insulating layer possesses the necessary mechanical strength,so that it is not crushed or deformed by any disproportional amountduring manufacture, laying-out and operation of the pipe.

Several embodiments of the thermally insulating layer can be envisaged,depending on its use.

In the following, three examples of the use of the armoured pipeaccording to the invention will be described:

EXAMPLE 1

An oil well delivers a fluid, which consists substantially ofhydrocarbons and CO₂. In that the oil in the well has a temperature of120° C., PVDF (polyvinylidenedifluoride) is selected for the liner, inthat PVDF is particularly suitable for use at high temperatures and hasa very low permeability to CO₂. However, the liner cannot providesafeguard against a too severe cooling of the oil, and for this reason alayer of PP (polypropylene) is extruded around the liner. In that PP hasmuch poorer barrier characteristics against CO₂ than PVDF, a destructivebuild-up of pressure will- not arise in the interface layer. At the sametime, the PP layer ensures that the thermal transport of heat betweenthe inside of the pipe and the armour layer is held at an acceptablelevel. Since the permeability of the PP layer decreases with thetemperature, a very thick PP layer will require to be provided withdefects, which will serve as transport paths for diffused gases.

EXAMPLE 2

An oil well delivers a fluid, which is substantially a mixture ofaromatic hydrocarbons and water. In that the temperature of the oil wellis low, PA 11 (polyamide 11) is selected as liner material. A polyketonis selected as thermal insulation material. In order to prevent theaccumulation of water in the interface layer, a series of small holes 11is established in the thermal insulation material, which ensures thatwater which is diffused from the inside of the pipe into the interfacelayer can escape.

EXAMPLE 3

An oil well delivers a fluid, which almost gives rise to the formationof wax, which means that a very high thermal insulation capability isdemanded for the pipe. Around the inner liner, which is made of PE(polyethylene), a thermally insulating multi-layer is applied, which isbuilt up of a layer of extruded, syntactic foam on which a layer ofsolid polypropylene is extruded. Because of the low temperature, thediffusion through the inner liner is not problematic, and thus nofurther measures are taken to prevent destructive build-up of gases inthe interface layers. Here, the thermally insulating layer is built upof two layers, i.e. a layer of syntactic foam, which has excellentthermal insulation characteristics, and an outer layer of PP, which haspoorer thermal characteristics but, on the other hand, possessesexcellent mechanical characteristics.

It will be obvious that the insulation can be configured in many moreways than described in the examples above, which merely serve toillustrate the multiplicity of possibilities, which are available forconfiguration of thermal insulation within the scope of the patentclaims.

1. Armored flexible pipe comprising an inner liner which surrounds acarcass, and where on the outer side of the inner liner there is apressure armor layer and a tensile armor layer which are shielded fromthe surroundings by an outer sheath, characterized in that athermally-insulating layer consisting of a homogenous polymer or apolymer mixture is provided with a number of small holes and ispermeable to fluids.
 2. Armored flexible pipe according to claim 1,characterized in that a part or the whole of the thermal insulationcomprises a polyolefin.
 3. Armored flexible pipe according to claim 1,characterized in that a part or the whole of the thermal insulationcomprises a polyketon.
 4. The process of transporting oil or relatedfluids, or of cold fluids comprising the step of transporting said fluidthrough a flexible pipe according to claim
 1. 5. An armored flexiblepipe comprising an inner liner which surrounds a carcass, and where onan outer side of the inner liner there is a pressure armor layer and atensile armor layer which are shielded from the surroundings by an outersheath, and further comprising a thermally-insulating layer that ispermeable to fluids and is provided with means which permit an adequateremoval of materials diffused to an interface layer between the innerliner and the thermally insulating layer.
 6. An armored flexible pipecomprising an inner liner which surrounds a carcass, and where on anouter side of the inner liner there is a pressure armor layer and atensile armor layer which are shielded from the surroundings by an outersheath, further comprising a thermally-insulating layer that ispermeable to fluids and comprises two or more layers, wherein two layersconsist essentially of a syntactic foam and of polypropylene.